Some  Abnormal  7/ater  Rela- 
tions in  Citrus  Trees  of  the 
Arid  Southwest  and  Their 
Possible  Significance. 
By  Robert  W.  Hodgson. 
Agr.  Sci.  Sept.  29,  1917. 
x 3rd  Copy 


UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

IN 

AGRICULTURAL    SCIENCES 

Vol.  3,  No.  3,  pp.  37-54,  plate  12  September  29,  1917 


SOME  ABNORMAL  WATER  RELATIONS  IN 

CITRUS  TREES  OF  THE  ARID  SOUTH- 

WEST  AND  THEIR  POSSIBLE 

SIGNIFICANCE 


BY 

ROBERT  W.  HODGSON 


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UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

IN 

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Vol.  3,  No.  3,  pp.  37-54,  plate  12  September  29,  1917 


SOME  ABNORMAL  WATER  RELATIONS  IN 
CITRUS  TREES  OF  THE  ARID  SOUTH- 
WEST AND  THEIR  POSSIBLE 
SIGNIFICANCE 


BY 

EGBERT  W.  HODGSON 


INTRODUCTION 

The  progress  of  the  development  of  the  citrus  industry,  in  general, 
and  that  of  California  in  particular,  has  frequently  been  retarded  or 
temporarily  stopped  by  serious  obstacles  in  the  form  of  insect  pests 
or  plant  diseases.  Some  of  the  most  baffling  of  these  troubles  fall 
naturally  into  a  group  which  for  want  of  a  better  name  has  come  to 
be  known  as  that  of  ' '  physiological  diseases, ' '  which  are  thought  to  be 
caused  by  various  obscure  derangements  of  nutrition  or  other  vital 
functions.  This  group  includes  mottled-leaf,  die-back,  chlorosis,  June 
drop,  puffing  of  the  fruit,  and  others  of  less  importance.  Knowledge 
of  the  true  nature  of  this  class  of  diseases  is  extremely  meager  in  spite 
of  the  fact  that  they  have  received  much  earnest  attention  from  scien- 
tific investigators ;  and  little  can  be  accomplished  in  the  way  of  devis- 
ing control  measures  until  much  more  is  known  in  regard  to  them. 
Nor  can  we  hope  to  progress  far  beyond  the  realm  of  speculation  with- 
out greatly  augmenting  our  knowledge  of  the  physiology  and  anatomy 
of  the  normal  citrus  tree  when  grown  under  any  one  of  a  series  of 
very  widely  varying  environmental  complexes  which  obtain  in  differ- 
ent parts  of  the  arid  southwest. 

It  is,  therefore,  proposed  to  attempt  by  means  of  a  series  of  sys- 
tematic experimental  studies  to  obtain  some  definite  and  accurate 
information  on  the  physiology  of  the  genus  Citrus.  It  is  hoped  that 
the  results  may  serve  as  a  basis  for  the  elucidation  of  some,  at  least, 


38  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

of  the  important  problems  referred  to  above.  The  studies  in  ques- 
tion will  attempt  to  shed  light  on  transpiration  problems,  nutrition 
problems,  and  others  equally  important.  The  paper  which  is  sub- 
mitted herewith  forms  an  introductory  contribution  to  the  subject 
under  investigation. 

The  writer  is  not  unaware  of  the  essential  similarity  between  the 
physiological  problems  presented  by  citrus  and  other  fruit  trees.  He 
has  chosen,  however,  to  study  the  physiology  of  the  citrus  tree  as  a 
separate  entity  because  of  the  reasons  given  above,  and  the  further 
one  that  the  peculiar  climatic  conditions  under  which  this  tree  is 
frequently  placed  in  the  arid  southwest,  demand  a  special  treatment. 
Doubtless  much  may  be  gained  from  these  studies  which  will  apply 
to  physiological  problems  connected  with  other  trees. 

The  data  here  presented  were  obtained  during  an  investigation  of 
one  of  the  so-called  physiological  diseases  above  mentioned,  namely, 
the  June  drop.1  Ever  since  the  Washington  Navel  orange  has  been 
grown  in  the  dry  interior  valleys  of  Arizona  and  California,  this 
variety  has  been  subject  to  excessive  dropping  of  the  young  fruits. 
This  has  came  to  be  known  popularly  as  the  June  drop  although  the 
fall  of  the  fruits  is  by  no  means  confined  to  June  but  may  occur  at 
any  time  from  petal  fall,  in  April,  until  the  fruit  reaches  several 
inches  in  diameter  in  August.  The  prevalence  and  amount  of  this 
dropping  seems  to  be  influenced  to  a  marked  degree  by  certain 
environmental  factors  to  which  the  trees  are  subject.  The  regular 
annual  shedding  of  the  young  fruits  is  most  serious  in  regions  where 
the  annual  precipitation  is  lowest,  the  mean  summer  temperature 
highest,  atmospheric  humidity  lowest,  solar  radiation  most  intense, 
and  air  movement  greatest  during  the  growing  season.  That  the 
excessive  drop  of  young  fruit  is  in  some  way  intimately  connected 
with  extreme  climatic  conditions  is  indicated  by  the  fact  that  in  some 
parts  of  southern  California,  where  the  drop  is  ordinarily  not  excess- 
ive, the  hot  wave  of  June  15-17,  1917,  during  which  a  temperature 
of  118°  F  was  experienced  in  the  Riverside  and  Redlands  districts, 
was  immediately  followed  by  a  drop  so  severe  that  practically  the 
entire  young  crop  of  navel  oranges  was  lost. 

The  experimental  work  from  which  the  data  were  obtained  was 
carried  on  at  Edison,  Kern  County,  California.  Edison  comprises 


i  This  investigation,  which  is  now  in  progress,  was  carried  on  in  collabora- 
tion with  Professor  J.  Eliot  Coit  who  planned  the  first  series  of  experiments 
and  began  the  work  in  February,  1916.  A  joint-authorship  paper  correlating 
this  and  other  aspects  of  the  June  drop  phenomenon  is  in  course  of  prepara- 
tion. 


1917]  Hodgson:  Abnormal   Water  Relations  in  Citrus  Trees  39 

a  small  colony  of  about  seven  hundred  acres  of  orange  orchard 
located  eight  miles  southeast  of  Bakersfield  and  surrounded  on  two 
sides  by  typical  desert  of  the  southern  San  Joaquin  valley,  with  its 
characteristic  semixerophytic  flora.  Extreme  climatic  conditions,  as 
above  mentioned,  are  operative  there  but  the  Washington  Navel 
orange  matures  early  and  is  of  excellent  quality,  although  crops  are 
small  because  the  drop  referred  to  is  excessive. 

WATER  RELATIONS  AND  ABSCISSION 

It  has  long  been  recognized  that  abnormalities  or  irregularities  in 
the  water  relations  of  plants  are  often  associated  with  the  abscission 
of  various  plant  parts.  Balls2  was  able  to  cause  complete  shedding 
of  leaves,  flower  buds,  and  bolls  of  the  cotton  plant  Gossypium  hcr- 
baceum  within  four  days  by  pruning  the  roots  and  so  limiting  the 
ability  of  the  plant  to  take  up  water.  Lloyd3  in  his  investigation  of 
the  cause  of  abscission  in  the  same  plant  came  to  the  conclusion  that 
the  causative  factor  lay  in  a  steady  decrease  in  the  moisture  content 
of  the  soil  in  contact  with  the  roots  of  the  plant.  This  reduction 
causes  a  severe  tax  on  the  power  of  the  plant  to  maintain  normal 
water  relations  and  results  in  fluctuations  in  the  water  content  of 
the  aerial  parts  which,  in  turn,  leads  to  abscission. 

Although  the  work  of  Lloyd  was  performed  in  the  humid  southern 
states,  he  makes  the  statement  that  "there  seldom  occurs  a  day  on 
which  there  is  no  minus  water  fluctuation  in  the  plant."  He  based 
this  conclusion  not  only  on  data  derived  from  shedding  records  but 
also  on  a  study  of  transpiration  rates,  and  water  deficit  in  the  leaves. 
In  connection  with  his  observations  on  the  effect  of  temperature  in 
causing  acceleration  of  abscission,  he  came  to  the  conclusion  that  ' '  the 
water  deficit  is  the  cause  of  the  rise  of  temperature  in  the  tissues  and 
that  this  constitutes  the  stimulus  which  directly  leads  to  abscission." 

Other  evidence  of  the  occurrence  of  marked  deficits  in  the  water 
content  of  plant  organs  is  not  lacking.  Livingston  and  Brown,4  work- 
ing with  a  number  of  plants  growing  near  Tuscon,  Arizona,  found 
that  (with  the  exception  of  the  true  xerophytes  as  Covillea  and 
Prosopis)  during  the  afternoon  the  leaves  suffered  a  marked  decrease 
in  water  content  which  was  made  up  during  the  night.  This  periodic 


2  Cairo  Sci.  Jour.,  vol.  5,  p.  221,  1911. 

3  Trans.  Eoyal  Soc.  Can.,  ser.  3,  vol.  10,  p.  55,  1916,  see  also  Bull.  Torr.  Bot. 
Club,  vol.  40,  p.  1-26,  Jan.,  1913. 

4  Bot.  Gaz.  vol.  53,  p.  319,  April,  1912. 


210043 


40  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

diurnal  condition  of  dessication  has  been  found  by  Livingston  and 
Brown  to  serve  as  a  check  on  the  absolute  transpiration  and  has  been 
termed  "incipient  drying."  Lloyd5  independently  obtained  similar 
results  in  his  investigations  on  Fouquieria  splendens  and  Mrs.  Shreve6 
established  the  same  phenomonon  in  1913  with  Parkinsonia  micro- 
phylla. 

Inasmuch  as  the  genus  Citrus  is  undoubtedly  a  mesophyte  of 
tropical  origin  and  therefore  grown  in  the  interior  valleys  of  Cali- 
fornia under  purely  artificial  conditions,7  it  would  naturally  be 
expected  that  the  abnormal  water  relations  above  discussed  might 
obtain  to  an  unusual  degree,  especially  during  the  hot  growing  period, 
when  the  ability  of  the  plant  to  make  up  for  excessive  transpiration 
is  taxed  to  the  limit.  Citrus  fruits  are  borne  on  wood  of  the  current 
season's  growth  which  ordinarily  bears  six  to  eight  leaves  on  the  same 
fruiting  shoot.  Therefore,  it  seemed  reasonable  that  under  conditions 
of  excessive  transpiration  the  leaves  might  draw  on  the  water  supply 
of  the  fruits  and  thus  bring  about  an  abnormal  water  relation.  With 
the  above  considerations  in  mind  it  occurred  to  the  writer  that  this 
premature  fall  of  the  fruits  might  be  due  to  irregularities  or  abnormal- 
ities in  the  water  relations  between  the  fruits  and  foliage,  resulting  in 
abscission  in  some  way  analagous  to  the  shedding  of  cotton  bolls  under 
the  stimulus  of  a  water  deficit. 

The  method  used  in  obtaining  the  data  here  presented  consisted 
in  the  main  of  simple  moisture  determinations  of  leaves  and  fruits  of 
various  kinds  taken  at  different  hours  of  the  day.  The  material  was 
gathered  and  quickly  placed  in  weighing  cups  fitted  with  ground 
glass  covers.  After  weighing,  the  material  was  thoroughly  dried 
and  then  reweighed.  For  convenience  in  the  case  of  fruits  and  large 
leaves,  the  material  was  cut  into  small  pieces.  The  calculations  are 
based  on  the  dry  weight  of  the  material,  except  as  otherwise  stated. 
The  data  obtained  are  shown  in  condensed  form  in  table  1.  The 
figures  shown  represent  averages  of  at  least  ten  duplicate  determina- 
tions, and  in  most  instances  of  more. 

The  data  presented  in  table  1  show  some  very  interesting  condi- 
tions. It  is  quite  clear  that,  with  the  exception  of  the  new  succulent 
growth,  the  young  fruits  are  at  all  times  higher  in  water  content  than 
the  leaves  situated  near  them.  These  data  also  seem  to  leave  no  doubt 


B  Plant  World,  vol.  15,  p.  11,  1912. 

e  Ann.  Ept.  Dir.  Bot.  Res.  C.  I.  W.,  Feb.  12,  1913,  p.  81. 

7  For  a  more  complete  discussion  see  Livingston,  B.  E.,  "A  single  index  to 
represent  both  moisture  and  temperature  conditions  as  related  to  plants. '' 
Physiological  Researches,  vol.  I,  No.  9,  April,  1916. 


Hodgson:  Abnormal  Water  Belations  in  Citrus  Trees  41 

TABLE  1 
AVERAGE  MOISTURE  CONTENT 

Average  water  content 

in  per  cent 
Kind  of  material  (Dry  weight) 

New  leaves  about  two  weeks  old  ................................................  242.0 

Full  grown  leaves  of  current  season  's  growth  ........................  162.2 

Leaves  of  one  season's  growth  —  about  one  year  old  ............  132.7 

Leaves  of  two  season's  growth  —  about  two  years  old  ........  126.1 

Leaves  of  three  or  more  season's  growth.  Over  two  years 

old    ................................................................................................  117.6 

Leaves  of  current  season's  growth.  Gathered  between 

9  A.M.  and  12  P.M  .......................................................................  164.9 

Same  gathered  between  1  P.M.  and  4  P.M  .................................  157.2 

Leaves  of  current  season  's  growth  gathered  from  behind 

fruits  between  9  A.M.  and  12  M  .............................................  166.8 

Same  gathered  between  1  P.M.  and  4  P.M  .................................  160.4 

Fruits  destined  to  subsequent  abscission,  one-third  to  three- 

fourths  inch  in  diameter  ........................................................  191.5 

Fruits  apparently  normal  gathered  between  9  A.M.  and  12  M.s  260.2 

Same  gathered  between  1  P.M.  and  4  P.M  ...................................  247.7 

Fruits  destined  to  subsequent  abscission  gathered  between 

9  A.M.  and  12  M  .......................................................................  201.4 

Same  gathered  between  1  P.M.  and  4  P.M  .................................  179.2 


of  the  fact  that  as  the  leaves  grow  older  there  is  a  progressive  decrease 
in  water  content. 

It  is  also  quite  evident  that  a  regular  diurnal  decrease  in  the  water 
content  of  leaves  of  the  current  season's  growth  is  manifest  during 
the  afternoon.  Such  leaves  averaged  164.9%  in  water  content  for  the 
period  between  9  A.M.  and  12  M.  and  only  157.2%  for  the  period 
between  1  P.M.  and  4  P.M.  This  difference  does  not  appear  significant 
when  viewed  in  the  light  of  the  large  differences  obtained  by  Living- 
ston and  Brown  with  some  of  their  material.  However,  it  should 
be  borne  in  mind  that  those  authors  were  dealing,  for  the  most  part, 
with  much  more  succulent  plants  containing  a  large  amount  of  water 
storage  tissue.  Further,  it  should  be  noted  that  these  figures  are 
averages,  since  the  determinations  on  which  they  are  based  were  not 
made  at  the  same  hours.  Individual  pairs  of  determinations  fre- 
quently showed  differences  of  as  much  as  25%  to  30%  in  as  short  a 
period  as  six  hours.  On  June  5  at  2  :30  P.M.,  with  the  temperature  at 
95°  F  and  the  relative  humidity  at  19%,  the  water  content  of  leaves 
of  the  current  season's  growth  was  144.3%.  At  4  A.M.  the  next  morn- 


s  The  fruits  used  for  these  determinations  averaged  a  little  larger  than 
those  gathered  in  the  forenoon  and  therefore  would  normally  be  somewhat 
higher  in  water  content. 


42  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

ing,  with  the  temperature  at  62°  F.  and  the  humidity  54%,  the  water 
content  of  similar  leaves  was  found  to  be  172.6%,  showing  a  differ- 
ence of  28.3%.  This  phenomenon  is  taken  to  indicate  the  presence  of 
incipient  drying  in  citrus  and  is  in  full  accord  with  the  results  of 
the  writers  above  mentioned  as  well  as  with  those  obtained  by  Lloyd. 

Since  the  young  fruits  have  a  higher  water  content  than  adjoin- 
ing leaves  which,  in  turn,  exhibit  a  diurnal  decrease  in  relative  water 
content,  the  conclusion,  a  priori,  that  the  leaves  might  possibly  draw 
on  the  water  supply  of  the  fruit  during  periods  of  excessive  transpira- 
tion seemed  entirely  plausible.  If  such  is  the  case  it  would  seem  that 
leaves  so  favorably  situated  should  not  show  this  daily  variation,  at 
least  to  the  degree  shown  in  the  leaves  not  so  favorably  situated.  The 
data  in  table  1  show,  however,  that  the  average  difference  in  water 
content  of  the  two  sorts  of  leaves  gathered  in  the  forenoon  and  after- 
noon is  quite  small.  This  is  taken  to  indicate  that  if  such  leaves  do 
utilize  the  water  supply  of  the  fruits,  the  evaporating  power  of  the 
atmosphere  is  so  strong  that  as  fast  as  they  receive  this  surplus  water, 
it  is  lost  and  thus  causes  no  appreciable  difference  in  their  relative 
water  content. 

The  next  step  was  to  ascertain  the  water  content  of  different  kinds 
of  fruits,  those  destined  to  remain  and  mature,  and  those  showing 
indications  of  subsequent  abscission.  It  is  quite  easy  to  distinguish 
between  the  two,  from  a  week  to  ten  days  before  abscission  occurs, 
by  the  difference  in  their  appearance.  Exposed  fruits  destined  to 
drop  exhibit  a  small  yellow  spot  about  the  navel  end  several  weeks 
before  the  actual  drop  occurs.  This  spot  gradually  extends  and 
spreads  until  at  abscission  it  usually  occupies  at  least  half  the  area 
of  the  fruit.  In  the  case  of  well-shaded  fruits,  the  yellow  color  is 
evenly  distributed  over  the  entire  surface.  A  large  number  of  mois- 
ture determinations  were  made  which  showed  that  those  fruits  destined 
to  subsequent  abscission  averaged  59%  less  water  than  those  fruits 
destined  to  remain  and  mature.  (See  table  1.)  The  presence  of  this 
condition  in  the  fruits,  especially  when  considered  in  connection  with 
the  daily  increase  at  certain  hours  in  the  water  deficit  of  the  leaves 
immediately  behind  them,  seems  to  point  to  the  possibility  of  the  leaves 
depriving  the  fruit  of  a  part  of  their  normal  water  supply.  It  cer- 
tainly indicates  an  abnormal  water  relation. 

Lemon  growers  prune  their  trees  at  all  seasons  of  the  year,  even 
while  the  fruit  is  still  on  the  trees.  It  is  a  well  established  practice  to 
gather  the  good  fruit  from  the  excised  branches  immediately,  in  order 
to  prevent  it  from  becoming  flaccid.  Inasmuch  as  the  fruit,  as  ordi- 


1917]  Hodgson:  Abnormal  Water  Eelations  in  Citrus  Trees  43 

narily  picked  from  the  tree,  remains  turgid  for  several  months,  it  is 
the  common  belief  that  the  leaves  draw  the  water  out  of  the  fruit  when 
the  branch  is  severed  from  the  tree.  That  this  is  exactly  what  does 
occur,  when  the  leaves  are  deprived  of  their  normal  water  supply,  is 
shown  by  the  following  experiments: 

Experiment  1  —  Two  shoots  bearing  small  terminal  oranges  of 
approximately  the  same  size  and  having  the  same  number  of  leaves 
and  approximately  the  same  leaf  area,  were  taken  to  the  laboratory, 
placed  on  the  table  and  allowed  to  dry  under  similar  conditions  except 
that  in  one  case  the  fruit  was  severed  from  the  stem.  All  cut  surfaces 
were  sealed  with  vaseline. 

Within  twelve  hours  a  marked  difference  in  appearance  wras 
observed.  The  leaves  on  the  shoot  from  which  the  orange  was  detached 
were  considerably  shriveled  while  those  on  the  other  shoot  remained 
turgid  and  fresh.  This  difference  became  more  pronounced  as  time 
elapsed  and  in  thirty  hours  a  distinct  difference  in  the  appearance  of 
the  fruits  as  well  as  leaves  was  visible.  The  detached  fruit  remained 
firm  and  retained  its  dark  green  color  and  lustre  while  the  attached 
fruit  was  soft  and  flaccid  and  exhibited  a  dull  green  color  without 
lustre.  This  experiment  was  performed  repeatedly  with  both  oranges 
and  lemons  with  the  same  results.  (See  plate  12,  fig.  1.) 

As  all  the  cut  surfaces  were  sealed,  it  seems  clear  that  the  leaves 
on  the  shoot  with  fruit  attached  actually  drew  on  its  water  content 
and  that  it  was  this  supply  of  water  which  enabled  them  to  remain 
alive  and  fresh  long  after  the  leaves  on  the  other  shoot  had  withered 
and  died. 

Experiment  2 — Quantitative  data  on  water  content  were  desired 
to  substantiate  the  visible  indications  described  in  Experiment  1. 
Therefore  the  latter  was  repeated  several  times  and  moisture  determin- 
ations on  leaves  and  fruits  were  made  at  various  periods.  A  repre- 
sentative set  of  such  determinations  is  given  in  table  2 : 

TABLE  2 

MOISTURE  CONTENT  DETERMINATIONS,  TWENTY-FOUR  HOURS  AFTER  BEGINNING 

OF  EXPERIMENT  2 

Weight  of  Weight  of  Weight  of  Water  con- 
container  and  same  when  material  in  tent  per 

Kind   of  material                   fresh  material              dry  grams                      cent 
in  grams 

Orange  detached  from  branch 23.40  21.670  2.665  185.0 

Orange  attached  to  branch 23.585  22.367  2.075  142.1 

Leaves  from  branch                           (21.831  21.805  .181 

with  fruit  removed                           |22.045  22.010  .170  21.4  avg. 

Leaves  from  branch                           j  21.345  21.275  .175 

with  fruit  attached                           |20.604  20.477  .284  73.7  avg. 


44 


University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 


These  data  show  that  after  twenty-four  hours  the  leaves  on  the 
shoo't  with  orange  attached  contained  an  average  of  52.3%  more  water 
than  those  on  the  other  shoot.  They  further  show  that  the  detached 
fruit  contained  42.9%  more  water  than  the  attached  fruit  from  which 
the  leaves  had  been  drawing  their  supply.  This  is  considered  to  be 
conclusive  evidence  that  in  the  case  of  excised  branches  the  leaves 
can  draw  water  from  the  fruit. 

Experiment  3 — Two  shoots  in  every  respect  similar  to  those  used 
in  the  previous  experiments  were  treated  in  the  same  manner  as  those 
of  Experiment  1  and  2.  These  were  then  weighed  at  irregular  inter- 
vals until  they  had  reached  a  constant  weight.  During  the  interim 
they  were  kept  on  the  laboratory  table.  The  data  obtained  are  found 
summarized  in  table  3 : 

TABLE  3 

WATER  CONTENT  DETERMINATIONS  MADE  AT  IRREGULAR  INTERVALS  BASED  ON 
THE  WHOLE  WEIGHT 


Shoot   with    orange    attached 


Shoot  with  orange  detached 


Number     Weight 
of  hours    in  grams 
elapsed 

0           4.872 
3           4.436 

Loss  in 
grams 

Loss  in 
per  cent 

8.9 

Difference    Weight 
in         in  grams 
per  cent 

4.777 
.6           4.380 

Loss  in 
in  grams 

Loss  in  Difference 
per  cent          in 
per  cent 

.436 

.397 

8.3 

19 

3.957 

.915 

18.7 

3.830 

.947 

19.8 

1.1 

21 

3.895 

.977 

20.0 

3.742 

1.035 

21.7 

1.7 

24 

3.803 

1.069 

21.9 



3.607 

1.170 

24.4 

2.5 

26 

3.683 

1.189 

24.4 

3.442 

1.335 

27.9 

3.5 

27 

3.610 

1.262 

25.9 

3.342 

1.435 

30.0 

4.1 

44 

3.263 

1.609 

33.0 

2.911 

1.866 

39.1 

6.0 

49 

3.047 

1.825 

37.4 

2.682 

2.095 

34.8 

6.4 

51 

2.920 

1.952 

40.0 



2.575 

2.202 

36.0 

6.0 

91 

2.125 

2.747 

56.3 

2.008 

2.769 

57.9 

1.6 

96 

2.053 

2.819 

57.8 

1.960 

2.817 

58.9 

1.1 

99 

2.000 

2.872 

58.9 



1.935 

2.842 

59.5 

.6 

116 

1.921 

2.951 

60.5 



1.873 

2.904 

60.8 

.3 

119 

1.894 

2.978 

61.1 

1.852 

2.925 

61.2 

.1 

121 

1.881 

2.991 

61.3 

1.844 

2.933 

61.4 

.1 

140 

1.825 

3.041 

62.5 

.4 

1.807 

2.970 

62.1 

146 

1.797 

3.075 

63.1 

.5 

1.783 

2.994 

62.6 



162 

1.774 

3.098 

63.5 

.6 

1.770 

3.007 

62.9 

186 

1.736 

3.136 

64.3 

.8 

1.743 

3.034 

63.5 

195 

1.717 

3.155 

64.7 

.9 

1.726 

3.051 

63.8 

211 

1.705 

3.167 

65.0 

1.1 

1.720 

3.057 

63.9 



218 

1.695 

3.177 

65.2 

1.0 

1.710 

3.067 

64.2 



260 

1.666 

3.206 

65.8 

1.1 

1.686 

3.091 

64.7 

285 

1.652 

3.220 

66.0 

1.1 

1.675 

3.102 

64.9 

306 

1.642 

3.230 

66.2 

1.1 

1.664 

3.113 

65.1 

330 

1.631 

3.241 

66.5 

1.1 

1.652 

3.125 

65.4 

525 

1.613 

3.259 

66.8 

1.0 

1.631 

3.146 

65.8 

1917]  Hodgson:  Abnormal  Water  Relations  in  Citrus  Trees  45 

The  data  in  this  table  indicate  that  the  amount  of  water  in  the 
fruit  available  for  use  by  the  leaves  was  sufficient  to  maintain  the 
latter  alive  for  approximately  50  hours  after  the  shoot  was  cut  from 
the  tree.  It  is  further  evident  that  when  three  hours  had  passed  the 
leaves  on  the  shoot  with  fruit  attached  had  not  yet  begun  to  take  water 
from  the  fruit  to  any  appreciable  extent  because  the  shoot  with  fruit 
detached  shows  less  water  loss  than  the  shoot  with  fruit  attached. 
However,  this  condition  was  soon  reversed  and  the  leaves  began  to 


V 


20      40     60      80     100    120     140    160    180     200    220 

Fig.  1.  Showing  the  difference  in  per  cent  of  water  loss  of  shoot  with 
orange  attached  and  shoot  with  orange  detached.  The  water  loss  curve  of  the 
shoot  with  fruit  detached  is  considered  as  normal.  Ordinates  represent  differ- 
ences in  per  cent  of  water  loss,  abscissae,  the  time  elapsed  in  hours.  Water 
content  calculated  on  basis  of  fresh  weight. 


draw  on  the  water  in  the  fruit  while  the  leaves  to  which  no  water  was 
available  from  the  fruit  showed  indications  of  wilting. 

That  shortly  after  50  hours  had  passed  death  occurred  in  the 
leaves  of  the  shoot  with  fruit  attached  is  shown  by  the  rapid  increase 
in  the  amount  of  water  loss.  This  was  undoubtedly  due  to  increased 
permeability  of  the  cytoplasmic  cell  membranes  after  death.  After 
50  hours  the  difference  in  water  content  of  the  two  was  18.3%  in 
favor  of  the  shoot  with  fruit  attached.  However,  from  this  time  on 
until  both  had  reached  a  constant  rate  of  water  loss  (after  about  200 


46  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

hours),  this  shoot  lost  water  more  rapidly  than  the  shoot  with  fruit 
detached.  These  relations  are  very  clearly  shown  in  figure  1.  The 
normal  water  loss  curve  is  illustrated  in  figure  2. 

Experiment  4 — A  forked  twig  bearing  a  small  terminal  fruit  on 
each  branch  was  selected  and  cut.  The  fruits  were  immediately  im- 
mersed in  water  and  the  shoot  tied  to  a  support  in  such  a  fashion 
that  all  the  leaves  were  exposed  to  the  air,  the  fruits  alone  being 
immersed.  One  orange  was  now  removed  by  cutting  it  under  water 
and  all  cut  surfaces  were  sealed.  The  two  fruits  remained  under 


64 


56 


48 


32 


24 


50     100    150     200     250    300     350     400    450     500 

Fig.  2.  Showing  the  general  type  of  water  loss  curve  of  a  shoot  detached 
from  the  tree,  including  detached  orange.  Ordinates  represent  water  loss  in 
per  cent  and  abscissae,  the  time  elapsed  in  hours. 


water.  The  container  and  support  were  then  placed  on  a  bench  in 
the  shade  in  the  open  air  and  left  for  fifteen  hours,  at  the  end  of 
which  time  moisture  determinations  were  made  on  the  fruits. 

TABLE  4 
MOISTURE  DETERMINATIONS  AFTER  FIFTEEN  HOURS 


Kind  of 
Detached 

material 
orange 

Weight  of 
container  and 
fresh  material 
in  grams 

.  ..  24.680 

Weight  of 
same  when 
dry  in 
grams 

21.435 

Weight  of 
material  in 
grams 

3.330 

Water  con- 
tent per 
cent 

206.9 

Attached 

orange    .. 

..  23.210 

21.773 

2.570 

126.8 

1917]  Hodgson:  Abnormal   Water  Relations  in  Citrus  Trees  47 

The  data  in  table  4  show  that  at  the  end  of  fifteen  hours  there  was 
a  difference  in  water  content  between  the  two  fruits  of  80.1%.  There 
seems  to  be  no  way  of  accounting  for  this  large  difference  other  than 
that  the  leaves  had  actually  drawn  the  major  part  of  it  at  least,  from 
the  attached  fruit. 

WATER  TRANSPORT  STUDIES  BY  MEANS  OF  DYE  STUFF  SOLUTIONS 

Experiment  5 — Bearing  the  foregoing  findings  in  mind,  it  seemed 
desirable  to  determine  something  of  the  nature  of  this  reversal  of 
normal  water  flow  by  means  of  dye  solutions.  Accordingly  a  shoot 
bearing  a  terminal  fruit  was  cut  from  the  tree  and  the  orange  pared 
away  at  the  apical  end  to  open  the  tracheal  elements  and  admit  the 
dye.9  This  paring  was  done  under  the  solution  to  prevent  the  entrance 
of  air  bubbles.  Water  soluble  eosin  was  used.  The  orange  was 
immersed  in  the  liquid  for  a  half  hour,  after  which  the  shoot  was 
split  open.  The  tracheal  tubes  throughout  all  parts  of  the  leaves, 
stems  and  fruits  were  found  to  be  strongly  stained. 

Experiment  6 — It  seemed  desirable  to  simulate  the  actual  situation 
on  the  tree  as  nearly  as  possible  and  the  following  experiment  was 
designed  to  accomplish  this.  A  crooked  fruiting  branch  bearing  a 
number  of  small  lateral  shoots  and  leaves,  and  one  terminal  orange 
was  cut  under  water.  The  cut  end  was  kept  under  water  and  the 
branch  so  supported  that  the  fruit  was  immersed  in  an  eosin  solution. 
The  apex  of  the  orange  was  then  pared  as  described  above.  The 
branch  then  rested  with  its  basal  end  in  water  and  the  vascular 
bundles  of  the  fruit  open  to  eosin  at  the  other  end  of  the  branch. 
(See  pi.  12,  fig.  2.)  If  we  substitute  for  the  water  container  the  con- 
ducting system  of  the  tree,  and  for  the  watery  solution  of  eosin  the 
developing  fruit  high  in  water  content,  we  have  very  similar  conditions 
to  those  existing  in  the  experiment,  save  for  the  fact  that  the  fruit  is 
not  open  to  the  air  and  the  conducting  system  bears  a  certain  relation 
to  the  rest  of  the  tree. 

The  experiment  was  begun  late  in  the  afternoon  and  the  branch 
left  outdoors  over  night.  At  8  o'clock  the  next  morning  the  leaves 
were  examined  and  found  to  be  very  fresh  and  turgid.  Indeed  they 
were  noticeably  much  fresher  in  appearance  than  they  had  been  the 
evening  before.  On  careful  examination  absolutely  no  trace  of  eosin 


s  It  should  be  stated  here  that  the  Washington  Navel  orange  is  in  reality 
a  double  fruit,  with  a  small  secondary  orange  within  a  large  primary  fruit. 
This  interior  fruit  constitutes  what  is  known  as  the  navel  and  it  possesses  an 
independent  vascular  system  of  its  own  which  traverses  the  central  pith  of 
the  primary  fruit  before  ramifying  through  the  secondary  orange.  This  central 
pith  thus  acts  as  the  stem  to  the  small  fruit. 


48  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

could  be  found  in  any  part  of  the  shoot.  Samples  of  leaves  for  mois- 
ture content  determination  were  taken  at  8 :30  A.M.  Although  remain- 
ing in  the  shade  the  leaves  showed  at  12 :15  P.M.  distinct  evidence  of 
wilting  and  upon  examination,  eosin  staining  was  found  in  the  petioles 
of  every  leaf  on  the  shoot,  with  the  exception  of  a  few  on  the  extremi- 
ties of  the  side  branches.  At  12:30  P.M.  samples  were  taken  for 
moisture  determination. 

TABLE  5 

MOISTURE  CONTENT  OF  LEAVES  TO  WHICH  WATER  WAS  AVAILABLE  FROM 

Two  SOURCES 

Weight  of  container    Weight  of  same     Weight  of  mate-     Average  water 

and  material  in  when  dry  in          rial  in  grams  content  in 

Hour  picked  grams  grams  per  cent 

8:30  A.M.  21.967  21.617  .562  

23.455  23.162  .480  160.8 

12:30  P.M.  21.914  21.659  .459  

21.552  21.345  .382  121.6 

The  data  obtained  in  this  experiment  show  that  during  the  night, 
when  the  draught  of  the  atmosphere  on  the  water  supply  was  low,  the 
leaves  were  able  to  maintain  themselves  in  a  normal  condition  by 
using  water  from  the  base  container.  This  is  evidenced  by  the  normal 
water  content  of  the  leaves  in  the  morning,  as  well  as  indicated  by  the 
fact  that  no  eosin  whatever  was  drawn  back,  although  the  vascular 
bundles  were  open  to*  its  entrance.  However,  as  the  atmospheric 
evaporating  power  increased  during  the  forenoon,  it  became  more  and 
more  difficult  for  the  leaves  to  obtain  requisite  amounts  of  water 
through  the  conducting  system  in  the  normal  way.  At  a  point  near 
12  M.  the  leaves  became  unable  to  obtain  enough  water  in  this  fashion 
and  they  began  to  draw  on  the  aqueous  solution  of  eosin.  The  shoot 
was  now  drawing  water  from  both  ends  to  satisfy  the  demands  of  the 
transpiring  leaves.  But  the  atmospheric  pull  for  water  became  so 
severe  that  even  this  double  supply  did  not  suffice  to  maintain  the 
water  content  of  the  leaves  at  normal.  The  water  deficit  began  to 
increase  and  continued  until  a  condition  of  actual  wilting  resulted. 
Between  8:30  A.M.  and  12:30  P.M.  the  leaves  decreased  in  water  con- 
tent by  39.2%,  although  both  ends  of  the  shoot  were  immersed  in 
water. 

From  the  evidence  above  presented  it  seems  clear  that  under  con- 
ditions favorable  to  rapid  transpiration  it  is  entirely  possible  for  the 
leaves  of  citrus  trees,  at  least,  to  draw  water  back  from  the  young 
fruits.  Moisture  determinations  of  fruits  under  such  conditions 
showed  a  considerable  decrease  in  water  content  and  indicated  that 


1917]  Hodgson:  Abnormal  Water  Relations  in  Citrus  Trees  49 

this  water  is  utilized  by  the  leaves.  Indeed,  almost  exact  quantitative 
results  were  obtained,  the  percentage  of  water  loss  of  the  fruit  being 
approximately  equal  to  that  gained  by  the  leaves.  (See  Experiment 
2.)  Dye  stuff  experiments  have  shown  that  there  exist  no  physical 
difficulties  for  such  procedure.  It  then  remained  to  determine  whether 
this  phenomenon  actually  occurred  in  fruits  on  the  tree.  For  this  pur- 
pose the  procedure  of  the  previous  experiments  was  used,  namely, 
moisture  determinations  and  water  transport  studies  by  means  of  dye 
stuff  solutions.  However,  the  evidence  here  is  not  so  conclusive,  as 
all  the  experimental  work  was  necessarily  performed  on  shoots  in 
situ,  which  introduces  a  number  of  uncontrollable  adventitious  vari- 
ables, as  has  been  well  pointed  out  by  Dixon10  in  his  criticism  of  trans- 
piration measurements  performed  on  shoots  detached  from  the  tree. 
The  influence  of  the  remainder  of  the  tree  is  admittedly  an  unknown 
quantity.  Nevertheless  it  is  believed  that  the  data  obtained  are 
strongly  indicative  of  conditions  as  they  exist. 

From  table  1  it  will  be  seen  that  there  exists  a  considerable  differ- 
ence in  water  content  between  those  fruits,  destined  to  remain  and 
mature  when  picked  before  and  after  noon.  Fruits  of  this  sort 
gathered  in  the  morning  averaged  12.5%  more  water  than  those 
gathered  in  the  afternoon.  As  is  mentioned  in  the  footnote  to  table  1 
this  difference  is  probably  considerably  smaller  than  it  should  be  on 
account  of  the  larger  size  of  the  fruits  used  in  the  afternoon  determin- 
ations. That  this  same  condition  obtains  in  those  fruits  destined  to 
subsequent  abscission  is  also  shown  in  table  1.  In  this  case  the  difffer- 
ence  is  22.2%.  There  are  several  ways  in  which  a  decrease  of  water 
content  in  the  fruit  can  be  explained :  ( 1 )  the  water  is  actually 
drawn  back  from  the  fruits  by  the  leaves;  (2)  the  normal  supply  to 
the  fruits  is  considerably  reduced  by  being  appropriated  by  the  leaves 
before  it  reaches  the  fruits ;  or  ( 3 )  the  transpiration  ratio  of  the  fruit 
to  the  leaves  is  markedly  increased.  If  the  fruit  possessed  no  stomata 
and  did  not  transpire,  the  first  condition  must  hold.  However,  the 
fruit  does  possess  stomata  in  some  numbers  and  is  actively  transpiring 
at  the  same  time  as  the  leaves.  Therefore  it  seemed  advisable  to  make 
a  cursory  comparative  study  of  the  number  of  stomata  per  unit  of 
area  on  the  fruits  and  leaves  and  also  of  the  ratio  of  transpiring  area 
of  the  fruits  and  leaves  situated  immediately  behind  them.  While 
the  young  fruit  possesses  stomata  even  before  the  style  is  exfoliated, 
stomatal  counts  showed  that  the  number  is  comparatively  small,  rang- 

10  Transpiration  and  the  ascent  of  sap  in  plants  (London,  MacMillan,  1914) 
pp.  120-125 


50  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

ing  between  50-100  per  square  millimeter  as  compared  to  300-450  per 
square  millimeter  on  the  leaves.  Measurements  of  the  leaves  situated 
within  six  inches  of  the  fruit  showed  that,  in  addition  the  leaf  area 
immediately  behind  the  young  growing  fruit  is  larger  than  the  area  of 
the  fruit  until  it  reaches  approximately  two  inches  in  diameter,  after 
which  falling  of  the  fruit  is  comparatively  rare.  Therefore,  it  seems 
highly  probable  that  the  transpiration  of  the  fruit  as  compared  to 
that  of  the  leaves  situated  immediately  behind  it  is  an  almost  negligible 
factor  and  it  appears  reasonably  certain  that  either  water  is  actually 
drawn  back  or  that  the  normal  supply  is  decreased. 

Considering  these  two  possibilities,  the  first  merits  more  considera- 
tion as  it  is  supported  by  proof  which,  though  not  absolute,  is  at  least 
presumptive  evidence  of  a  strong  enough  character ;  while  on  the  other 
hand  the  second  possibility,  agreeing  though  it  does  with  the  most 
recent  theory  on  sap  movements  in  plants  as  put  forth  by  Dixon,  is 
still  a  theoretical  consideration.  According  to  this  theory,  which  pos- 
tulates strong  tensions  existing  in  the  ascending  water  columns,  no 
assumption  of  an  actual  reversal  of  the  current  is  necessary  in  order 
to  explain  a  decrease  in  moisture  content.  During  normal  conditions 
the  relation  between  the  tensions  existing  in  the  water  columns  leading 
to  the  fruits  and  those  leading  to  the  leaves  is  such  that  both  organs 
receive  an  adequate  water  supply.  The  tension  existing  in  any  one 
of  these  water  columns  is  a  function  of  the  transpiring  force  existing 
in  the  transpiring  plant  organ  as  modified  by  atmospheric  conditions. 
Therefore,  as  these  transpiring  forces  vary,  the  tensions  vary.  Trans- 
piration from  the  leaves,  for  reasons  pointed  out  above,  is  subject  to 
much  greater  variation  than  that  from  the  fruits.  Therefore  during 
periods  when  evaporation  is  greatly  accelerated  the  tensions  in  those 
water  columns  leading  to  the  leaves  are  greatly  increased  and  as  a 
consequence  more  water  is  drawn  to  them.  As  the  source  of  supply 
in  the  conducting  system  is  practically  constant,  the  amount  in  the 
fruits  is  thereby  reduced  and  this  results  in  a  decrease  of  relative 
water  content  of  a  magnitude  conditioned  by  the  transpiration  of  the 
fruit. 

However,  it  should  be  noted  that  the  data  in  table  1  show  a 
decrease  in  absolute  water  content  of  the  fruit  of  15%  to  20%,  a  loss 
of  considerable  magnitude.  There  are  only  two  ways  in  which  such 
a  decrease  in  absolute  water  content  can  take  place:  (1)  the  water 
is  lost  by  transpiration  from  the  fruit,  or  (2)  it  is  drawn  back  by 
the  leaves.  But  since  the  fruit  possesses  a  very  small  stomatal  area 


1917]  Hodgson:  Abnormal  Water  Eelations  in  Citrus  Trees  51 

as  compared  with  the  leaves  and,  moreover,  it  is  highly  probable  that 
a  large  percentage  of  this  area  is  non-functional,  being  obstructed 
by  accumulations  of  a  resinous  nature,  there  is  small  likelihood  for 
absolute  loss  of  water  in  this  manner  to  the  extent  noted.  Hence 
there  seems  but  one  way  to  explain  it  and  that  is  by  movement  back 
from  the  fruits. 

Evidence  of  an  indirect  nature  pointing  to  the  same  conclusion 
lies  in  the  fact  that  there  are  some  indications  that  abscission  of  a 
certain  proportion  of  the  young  fruits  is  directly  due  to  the  influence 
of  hydrolysing  enzymes  secreted  by  certain  saprophytic  or  facultative 
parasitic  fungi  always  found  present  on  the  shriveled  style* and  fre- 
quently in  the  proliferations  of  the  navel.  Such  enzymes  in  order  to 
act  on  the  abscission  layer  must  be  drawn  back  through  the  vascular 
systems  of  the  fruit  into  the  pedicel  where  this  layer  is  located. 
Investigations  on  this  point  are  now  in  progress. 

Experiment  7 — Three  similar  fruits  were  selected  on  different 
parts  of  a  tree ;  on  one  of  the  lower  branches  in  the  shade,  at  a  height 
of  four  feet,  and  in  the  top  of  the  tree  in  full  sunlight.  At  noon  each 
fruit  was  pared  so  as  to  admit  entrance  of  a  solution  and  then  plunged 
quickly  into  a  small  vial  containing  a  watery  solution  of  eosin.  These 
vials  were  securely  tied  to  the  shoot  and  left  suspended  for  two  hours. 
At  the  end  of  that  time,  on  cutting  leaves  from  these  shoots,  eosin 
staining  was  found  in  the  vascular  systems  of  all.  On  examining  back- 
ward toward  the  tree,  eosin  was  found  as  far  back  as  thirty  centi- 
meters. This  experiment  was  repeated  a  number  of  times  both  at 
Edison  and  at  Riverside  and  uniformly  gave  the  same  results,  although 
much  less  marked  at  the  latter  place.  In  every  case  the  backward 
movement  of  the  eosin  solution  was  at  its  maximum  during  the 
afternoon. 

Cutting  the  ends  of  branches  in  situ  under  a  watery  solution  of 
eosin  was  tried  at  different  times  of  day  and  gave  similar  results. 
This  experiment  was  performed  at  Edison,  Riverside  and  Indio.  At 
the  latter  place,  with  the  temperature  at  116°  F  and  the  humidity  at 
8%  the  eosin  solution  traveled  backward  at  the  astonishing  rate  of 
30  cm.  per  minute  at  6  P.M.  Similar  results  were  obtained  using 
Eucalyptus  rudis  as  material.  In  fact  with  long  slender  poles  of 
Eucalyptus  tereticornis  at  Edison,  such  a  remarkably  rapid  backward 
flow  of  eosin  was  observed  (105cm.  in  one  minute)  in  the  afternoon 
as  to  compel  the  conclusion  that  after  all,  the  force  responsible  for 
this  movement  under  such  conditions  must  be  negative  pressure  pro- 


52  University  of  California  Publications  in  Agricultural  Sciences       [Vol.  3 

duced  by  rapid  transpiration  rather  than  a  mere  difference  in  osmotic 
concentration  of  the  solutions  involved.  Moreover,  negative  pressure 
produced  by  transpiration  would  seem  a  more  plausible  explanation 
of  the  results  obtained  by  Chandler11  with  tomato  plants  and  by  him 
attributed  to  osmosis.  For  as  Dixon12  has  pointed  out  ' '  it  is  quite  pos- 
sible for  the  solvent,  water,  to  be  in  a  state  of  tension,  i.e.,  at  a  negative 
pressure,  while  the  dissolved  substance  may  be  at  a  positive  pressure 
and  be  active  as  a  distending  force  in  the  cell. ' ' 

These  experiments,  it  is  believed  show  that,  during  the  afternoon 
at  least,  strong  negative  pressures  exist  in  the  water  columns  of  citrus 
trees  under  the  climatic  conditions  here  considered  and  that  the  young 
developing  fruits  are  deprived  periodically  of  a  part  of  their  water 
supply  by  excessive  transpiration  from  the  leaves. 

During  the  June-drop  period  of  1916  a  number  of  holes  were  dug 
in  various  parts  of  the  Edison  orchard  to  a  depth  of  six  feet  and 
moisture  determinations  made  at  various  depths  and  on  all  sides  of 
the  tree.  The  moisture  content  was  found  to  range  between  5%-6% 
just  before  irrigation  and  between  10%-12%  soon  after  irrigation.  A 
fruit  grower  observing  this  soil  and  the  vigorous  growth  of  the  trees 
would  hardly  conclude  that  the  trees  were  suffering  from  lack  of 
water.  However,  the  specific  effect  of  variations  in  the  moisture  con- 
tent of  the  soil  on  the  transpiration  rate  and  water  content  of  orange 
leaves  has  not  yet  been  carefully  determined.  While  there  is  little 
room  to  doubt  that  the  above-ground  complex  is  more  important  in 
influencing  transpiration  than  the  below-ground  complex,  «still  it  is 
entirely  possible  that  this  abnormal  water  deficit  in  the  leaves  and 
fruit  may  be  more  easily  induced  by  sudden  changes  in  the  climatic 
complex  under  conditions  of  a  deficient  moisture  supply  in  the  soil  or, 
what  amounts  to  the  same  thing,  an  inhibition  of  the  normal  absorption 
due  to  lack  of  sufficient  aeration  or  excessively  high  soil  temperatures. 
Nevertheless  evidence  is  not  lacking  that  marked  changes  in  air  tem- 
perature and  humidity  may  be  sufficient  to  cause  abscission  of  young 
fruits  even  though  the  soil  moisture  conditions  be  ideal.  Such  appar- 
ently is  the  explanation  of  the  heavy  drop  of  Washington  Navels 
already  referred  to,  which  occurred  over  most  of  the  citrus  districts 
immediately  following  the  excessive  temperatures  of  June  15-17,  1917, 
when  the  mercury  reached  110-120°  F  in  many  parts  of  the  citrus 
district  south  of  the  Tehachapi  mountains. 

11  Mo.  Sta.  Ees.  Bull.  14,  pi.  13. 

12  7,00.  cit.  p.  140. 


1917]  Hodgson:  Abnormal  Water  Relations  in  Citrus  Trees  53 


CONCLUSION 

This  paper  deals  with  one  phase  of  an  investigation  of  a  so-called 
physiological  disease,  June  drop  of  the  Washington  Navel  orange. 
An  excessive  dropping  of  the  young  fruits  has  been  experienced  for 
years  in  the  dry  interior  valleys  of  California  and  Arizona. 

An  abnormal  water  relation  is  found  to  obtain  periodically  in 
citrus  foliage  and  in  the  young  fruits  during  the  hot  growing  season 
in  these  regions.  A  diurnal  decrease  in  water  content  of  the  fruits 
occurs  during  the  afternoon  and  is  accompanied  by  a  considerable 
increase  in  the  water  deficit  of  the  leaves.  Negative  pressures  of  con- 
siderable magnitude  are  found  in  the  water  columns  of  citrus  trees 
under  these  climatic  conditions.  These  attain  their  maximum  during 
the  afternoon.  The  dropping  of  the  fruits  appears  to  be  most  severe 
where  the  above  mentioned  water  relations  are  most  abnormal. 

Inasmuch  as  in  the  case  of  certain  other  plants  the  abscission  of 
young  fruits  has  been  shown  to  be  due  to  abnormal  water  relations  it 
is  suggested  that  such  may  be  the  case  here. 

In  conclusion,  the  writer  wishes  to  acknowledge  his  indebtedness 
to  Professor  J.  Eliot  Coit,  under  whose  supervision  and  with  whose 
assistance  a  large  part  of  the  experimental  work  was  done;  also  his 
obligations  to  Professor  Charles  B.  Lipman  for  his  kindly  interest  in 
the  investigation  and  for  his  many  useful  suggestions,  and  to  the 
Edison  Land  and  "Water  Company  for  their  kind  and  courteous 
cooperation. 


PLATE  12 

Fig.  1.  Showing  extent  to  which  the  leaves  can  draw  on  the  water  in  the  fruit. 
Both  shoots  were  cut  at  the  same  time  and  had  approximately  the  same  leaf 
area.  All  cuts  were  sealed  with  vaseline.  The  fresh-appearing  leaves  on  the 
shoot  at  the  left  have  maintained  themselves  at  the  expense  of  water  in  the 
fruit.  Note  the  difference  in  reflection  of  light  from  the  two  fruits.  See 
Experiment  1. 

Fig.  2.  Photograph  illustrating  an  orange  shoot  so  arranged  as  to  be  able  to 
draw  water  from  one  end  and  eosin  solution  through  the  pared  apex  of  a  small 
fruit  at  the  other.  In  spite  of  this  double  supply  a  large  water  deficit  occurred, 
and  eosin  was  drawn  back  from  the  container  on  the  right  to  the  leaf  next  to  the 
water  container  on  the  left.  See  Experiment  6. 


[54] 

210043 


UNIV.   CALIF.    PUBL.   AGR.   SCI.   VOL.   3 


[HODGSON]   PLATE  12 


Fig.  1 


Fig.  2 


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as  a  Parent,  n,  by  T.  H.  Goodspeed  and  A.  H.  Ayres.    Pp.  273-292, 

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SB 

608  Hodgson  - 
C5H6  Some  abnor- 
cop.3  mal  water 

relations  in 

citrus  trees  of  the 
arid  Southwest. 


This  book  is  DUE  on  the  last  date  stamped  below 


i    ptftK  la  l?K 


Form  L-9-15m-7,'32 


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